A cataract is a cloudy or opaque region in the lens of the eye, which impairs vision due to a decrease in transparency of the eye lens crystalline material. A common treatment is to surgically remove the entire lens and replace it with an artificial lens, typically made of acrylic or other bio-compatible plastic material.
With the development of laser procedures, such as LASIK to help correct vision by reshaping the cornea of an eye, several other new laser eye procedures are being considered that concern photo-ablation of eye tissue.
For instance, U.S. Pat. No. 4,538,608, issued to L'Esperance, Jr. for “Method and Apparatus for Removing Cataractous Lens Tissue by Laser Radiation” teaches how to deliver laser energy into the anterior of the eye lens and scan the laser beam in order to photoablate cataractous tissue. This procedure was further developed by J. Bille (U.S. Pat. No. 5,246,435 “Method for Removing Cataractous Material”), who proposed a procedure of laser energy delivery to separate lamellae in the stroma by focusing a laser beam between lamellae layers and photoablating tissue at the interface between these layers.
These publications discussed using lasers having pulse lengths of several nanosecond duration (1 nsec=10−9 sec, a time in which light travels about 1 foot). With such pulse durations, each laser shot creates undesirable side effects, including strong shock waves within the eye, and significant tissue heating. In the late 1980's ultra-short lasers having pulse durations of less than 1 psec, where 1 psec=10−12 sec were developed. The total amount of ablation is a function of both power and the amount of energy delivered to the ablation area. The rate of ablation is, however, only a function of power, not energy, so these ultra-short pulses could produce finely controlled ablation while significantly reducing the undesirable side effects. Such laser have, therefore, beenconsidered for use in eye surgery. For example, T. Juhasz et al., U.S. Pat. No. 5,993,438 “Intrastromal photorefractive keratectomy”, T. Juhasz, U.S. Pat. No. 6,110,116 “Method for corneal laser surgery”, and T. Juhasz et al., U.S. Pat. No. 6,146,375 “Device and method for internal surface sclerostomy” teach using ultrashort (picosecond and femtosecond) laser pulses for more precisely cutting the so called “flap,” in LASIK surgery; employing a photodisruption technique for reshaping the cornea; and for using transcleral photodisruption of tissue on the interior surface of the sclera. Recently, U.S. Pat. No. 7,824,870 granted to Kovalcheck et al. on Nov. 2, 2010 entitled “System for dissociation and removal of proteinaceous tissue” teaches the removal of lens tissue using a high-intensity, electrical ultrashort-pulses between electrodes of the tip of a hollow surgical probe surrounding a volume of lens tissue.
However, in all the above cited patents ultra-short laser pulses, i.e., femtosecond pulses, are used with relatively low intensity, typically less than 10−9 W/cm2.
In contrast, the present invention uses ultra-short laser pulses of a very high intensity, in range of 1013-1015 W/cm2. This increased intensity significantly alters the nature of the interaction of the pulses with the material. This interaction has become, somewhat erroneously, termed multi-photon ablation to differentiate it from photo-ablation. In traditional photo-ablation, the laser pulse delivers energy that heats the electrons in the material to a sufficient extent to break molecular bonds in the material, freeing the molecules from the material that may, for instance, be the tissue of an eye lens.
In contrast, the process termed multi-photon ablation is a completely different method of material removal than photo-ablation. In multi-photon ablation, individual molecule absorb several photons almost instantaneously, in a timeframe that is faster than the molecule's or atom's relaxation time. This creates an ultra-high electric field in the vicinity of such molecules or atoms that frees them from the tissue being multi-photon ablated. Multi-photon ablation requires a very high laser pulse intensity, equal to or higher than 1012 W/cm2 and preferably in the range of 1013-1015 W/cm2. This causes a non-thermal ablation of matter, whereas other laser based ablation methods are thermal.
For example, a 5 mJ pulse with 50 femtosecond pulse duration focused down to a diameter of 10 to 100 μm provides a pulse intensity in the range of 1013-1015 W/cm2. At such intensities particles, such as molecules or atoms, at the surface of the target material, for instance tissue, are under a very high electric field, which may exceed the work force, or bonding, of a molecule or atom to the target such as tissue material, therefore freeing them from the target surface and creating the effect of ablation but practically without heating the target material. Initiating multi-photon ablation with a given laser pulse is based on a probability that is most affected by the pulse intensity. So, while multi-photon ablation may be possible below an intensity of 1012 W/cm2, the probability that a given pulse causes multi-photon ablation at lower intensities is significantly lower, and in practice, negligible. Furthermore, because of the probabilistic nature of multi-photon ablation, descriptions herein of the multi-photon ablation processes do not preclude the possibility that certain laser pulses within such processes may fail to invoke multi-photon ablation and that certain pulses may instead invoke photo-ablation.
In the present invention, the ultra-short laser pulses are directed such that when they reach a focal point within the eye lens, they are focused to such an intensity that at that position they interact with the lens material by the multi-photon processes, and disintegrate cataractous lens tissue by electrical field disruption of the molecular bonds. Such disintegration creates very little heat or shock. Once disintegrated, the lens tissue may be removed partially or fully through a channel formed to access the lens tissue, or by using another instrument such as, but not limited to, a needle or a syringe to evacuate the effluent material. The process has been applied to tattoo removal, cornea reshaping, and presbyopia corrections as detailed in corresponding U.S. Pat. Nos. 8,187,256; 8,382,744; and 8,596,281.
Explanations of multiphoton ablation and related phenomena may be found, for instance in U.S. Pat. No. 5,656,186 “Method for controlling configuration of laser induced breakdown and ablation”, issued to G. Mourou et al. the contents of which are incorporated herein in their entirety.
Specifically, Mourou teaches about the relationship between laser fluence threshold for breakdown and photoablation in tissue and laser pulse duration. Fluence (symbol F) is the term used in photochemistry to specify the energy delivered in a given time interval (for instance by a laser pulse) and it is usually measured as the number of Joules deposited per square cm over a certain period of time (J/cm2). Pulse duration is given the symbol τ, and is usually measured in psec. It is shown by Mourou et al. that, starting at a fluence level of F≈10, J/cm2 at a pulse duration τ≈10 nanosecond (nsec), F decreases as τ1/2 over the range from 10 nsec down to 10 picosecond, then decreases by a factor of two for pulse durations from 10 psec down to 1 psec, and then stays constant at F≈0.4 J/cm2 down to 100 fsec.
Therefore, for a pulse duration of τ≈10 nsec, the typical energy (E) required to ablate a surface area of diameter D≈100 micrometer (μm) is E≈1 mJ, whereas with a pulse duration of τ≈100-200 fsec (for such short pulses a typical D≈20 μm) the typical energy required is only E≈1.6 μJ. Hence, to photo-ablate tissue by the multi-photon process using fsec rather than nsec laser pulses, the energy levels required are more than 500 times smaller, and, therefore, shock waves may be negligibly weak for ultra-short pulse laser multi-photo ablation.
Various implementations are known in the art, but fail to address all of the problems solved by the invention described herein. Various embodiments of this invention are illustrated in the accompanying drawings and will be described in more detail herein below.